Switched mode power supplies are generally known. They include those which function according to the flow converter principle and those which function as flyback converters. In the case of the latter, magnetic energy is stored during a capture phase by means of a current which flows through the primary winding of a transformer. Said magnetic energy is delivered to a load, which is connected on a secondary side, during a subsequent release phase after rectification and smoothing. A switch element, which is connected in series with the primary winding of the transformer, is activated and deactivated by a control device depending on the load.
A switched mode power supply is normally used to connect a direct current load to an alternating current network. In this case, the switched mode power supply is connected to the alternating current network on the input side and, in a first step, a direct voltage is provided in an intermediate circuit by means of rectification and smoothing. If the switch element is activated, the intermediate circuit is then closed via the primary winding and a current flows which causes a magnetic flux in the core of the transformer. The electrical energy which is supplied in this capture phase is stored as magnetic energy in the core of the transformer. In this case, it is important to consider the so-called stray inductance of the transformer, which stray inductance cannot be used for the energy transfer. It represents the non-ideal coupling between primary and secondary winding. At the beginning of the release phase, i.e. when the switch element disconnects, the primary-side stray energy causes a high feedback voltage at the switch element, which can destroy the switch element. This behavior is critical above all in the case of powerful switched mode power supplies with large transformers. Here, the primary-side stray energy can no longer be simply thermally reduced in a snubber network, and instead the stray energy must be returned to the primary intermediate circuit, also for the purpose of increasing the efficiency.
In order to limit the feedback voltage, e.g. DE 35 37 536 A1 provides for the primary winding to include a tap which corresponds approximately to the maximal sampling ratio. The tap is connected to a capacitor, which is in turn connected to a reference potential via a diode. Each half-wave of the switched mode power supply is therefore clamped to zero or to a reference potential, such that no high feedback voltages can occur.
Young R.: “Stromversorgungen für SLICs”, Telecomm & Elektronik, 2001, volume 1, pages 27-32, describes a solution for avoiding excessive feedback voltage by using a snubber circuit which is arranged between primary winding and switch element. In this case, part of the primary-side stray energy is expended over a snubber resistor and a snubber capacitor, in order thus to limit the maximal feedback voltage and to avoid damaging the switch element. However, the efficiency of the power supply is also decreased thus.
JP 11098832 describes a flyback converter comprising a snubber circuit with a capacitor, a diode and an auxiliary winding. Although the losses can be restricted here, they still cannot be ignored in the case of powerful switched mode power supplies.
According to the prior art, a primary-side discharge circuit is therefore provided for powerful switched mode power supplies. This discharge circuit has the effect that the primary-side stray energy can be returned to the intermediate circuit via a primary-side discharge winding, without overloading the switch element in this case. The discharge circuit includes a diode which prevents a reverse current flow during the capture phase.
Dixon L.: “Transformer and Inductor Design for Optimum Circuit Performance”, Dallas, Tex., 2003, Texas Instruments Incorporated, pages 2-3 describes a method in which the primary winding is used as a discharge winding for returning the primary-side stray energy to the intermediate circuit. In the case of this arrangement (see also FIG. 1), the primary winding is attached by means of two additional interfaces to the intermediate circuit voltage and the reference potential of the intermediate circuit, wherein diodes which only allow a current return flow during the release phase are arranged in these interface connections. The connection of the primary winding to the intermediate circuit voltage and the reference potential during the capture phase is effected via two switch elements which are synchronously activated and deactivated by a controller.
U.S. Pat. No. 4,754,385 also describes such a circuit. In this case, a capacitor and parallel resistor are additionally connected in series with a diode. The capacitor buffers the backflow energy during the release phase in case the intermediate circuit voltage features a ripple wherein, during the lower ripple peaks, the intermediate circuit voltage drops below the voltage which is transferred from the secondary side to the primary side.
JP 60148374 A likewise describes a flyback converter with a primary winding which is additionally attached via diodes to the intermediate circuit voltage and the reference potential. In this case, a capacitor is provided in series with a diode, energy being buffered in said capacitor during short-term backflow phases.
A further known method for feedback of the primary-side stray energy into the intermediate circuit utilizes an additional primary-side auxiliary winding as a discharge winding. This auxiliary winding is wound close to the primary winding, wherein the winding direction is the same as that of the primary winding for optimal coupling, but the configuration is counter to the primary winding (see also FIG. 3). In the release phase, the primary-side stray energy is then returned into the intermediate circuit via the auxiliary winding and a diode.
In the case of switched mode power supplies, operating states can occur in which the input voltage and hence also the intermediate circuit voltage drop (e.g. in the event of a short-term power failure). In this case, the problem occurs that energy which could be utilized on the secondary side is returned to the intermediate circuit if the intermediate circuit voltage falls below the secondary voltage which has been transformed in the demagnetized winding in accordance with the turn ratio. In the case of lower input voltages, therefore, the prior art provides for utilizing a so-called power derating, irrespective of the thermal conditions. In this case, a lower power that can be unloaded is defined for smaller input voltages.
Generally limiting the function of a switched mode power supply to higher minimal input voltages is also known. The range including low input voltages is not fully utilized, therefore, since it can result in a rapid destruction of the discharge circuit.